1. Field of the Invention
The present invention relates generally to endoluminal tubular stents, such as stents and other structures. More particularly, the present invention provides modular tubular stent structures having properties which can be tailored for individual body lumens, including blood vessels, and more particularly, for placement in the venous system.
Stents and related endoluminal devices are currently used by medical practitioners to treat portions of the aorta and peripheral arterial vascular system that have become weakened, developing an aneurysm (a weakening of the artery wall resulting in a distended arterial section that is subject to rupture), or that have become, or portions of the venous vascular system that have become so narrowed that blood flow is restricted (commonly referred to as “stenosis”).
Stents are generally cylindrically shaped devices which function to expand when deployed. Stents may be balloon expandable or self expanding. The balloon expandable stent is a stent that is usually made of a coil, mesh or zigzag design. The stent is pre-mounted on a balloon and the inflation of the balloon plastically expands the stent with respect to the balloon diameter. Self-expanding stents are a tubular device stored in an elongate configuration in what is called a delivery system or applicator. The applicator is introduced percutaneously into the body into a vessel at a suitable location, and guided through the vessel lumen to the location where the stent is to be released. The delivery system and the stent are often provided with radiological markers with which the positioning and the release of the stent can be monitored in situ under fluoroscopy. Upon release, the stent material auto expands to a predetermined size.
Commonly used self-expanding stents are braided stents, or laser cut stents. A braided stent is a metal stent that is produced by what is called a plain weaving technique. It is composed of a hollow body, which can stretch in the longitudinal direction and whose jacket is a braid made up of a multiplicity of filament-like elements which, in the expanded state of the braided stent, intersect a plane, perpendicular to the longitudinal direction, at a braid angle. A braided stent undergoes a considerable change in length when deployed (“foreshortening”), this change in length being all the greater the greater the original diameter and the smaller the original braid angle (e.g. Wallstent from Boston Scientific (Boston, Mass. USA)). Because of the considerable shortening that takes place upon release of a braided stent, precise placement is difficult. Laser cut stents are constructed from a tube of material (most frequently, nitinol (a nickel titanium alloy), and also stainless steel, cobalt, etc) that is laser-cut during production to create a meshed device. The tube is comprised of sequential aligned annular rings that are interconnected in a helical fashion. The tube is compressed and loaded into the delivery device and expands to original size when released. Nitinol, which has thermal memory, may help stents made of this material expand into position when exposed to body temperature after delivery. Compared with self-expanding braided stents, laser cut stents provide more accurate stent deployment with less foreshortening. Laser cut stents are much less subject to foreshortening and are probably less rigid than braided stents.
For treatment of aneurysms, the stent generally includes a graft or liner, or may be a liner alone with a stent-like device at each end of the stent for sealing against healthy aortic vessel walls away from the weaken aortic section. The graft or liner is generally made of inelastic non-expanding material, and is generally impermeable to blood, as the stent-graft is intended to prevent blood flow through the liner and into the surrounding artery. Graft material is generally a non-self supporting fabric material that must be attached to the stent frame for support. The combination of a liner and a stent limits the possible radial expansion of the stent, as the liner material is generally inelastic. Constrained expansion of a stent by the liner is desired in the aortic system, as it is not desired to have the liner-stent seal against unhealthy distended aortic tissue—rather, the liner stent is deployed to create a sealed passage through the weakened aortic section and to prevent blood flow between the exterior surface of the liner/stent combination (or liner alone) and the weaken aortic walls, thereby preventing the possible rupture or bursting of the weakened aortic walls.
In the venous system, the setting is different—the issue is not weakened vessel walls, but stenosis within the vessel. Venous stenosis may be caused by clotting, scarring following blood clots or by focal external compressive forces on a venous vessel (such as in the femoral vein where it crosses the inguinal ligament or in the pelvic vein where it is crossed by overlaying pelvic arteries). In treating a venous stenosis, a liner or graft is not necessary; indeed, a liner or graft is not preferred, as the stent must function to expand against the narrowed vein section, thereby expanding the narrowed section to a more normal cross-sectional area. It is not desired to limit expansion a stent used for treating stenosis in the venous system.
While stents in the venous system are most often used to “prop open” blood vessels, they can also be used to reinforce collapsed or narrowed tubular structures in the body, such as the respiratory system, the reproductive system, or any other tubular body structure. These stents are generally mesh-like so that endothelial and other cells can grow through the openings integrating the stent into the venous wall and sometimes resulting in restenosis of the tubular structure. Inclusion of liners or grafts would prevent the integration of the stent into the venous wall. Typically, one or both ends of the stent is flared in order to facilitate anchoring within the vessel.
Most stents are designed to work in fairly small lumens and are relatively short in length. However, lumens in the venous system can be much larger than aorta and peripheral arteries and the desired stent length can be long in comparison to arterial stents. Both features of the venous system present problems for conventional stent design, where the conventional stent structure is typically formed with cylindrical frames having axially constant diameters and constant expansive forces along their lengths. Additionally, long length stent structures may also encounter variations in lumen size over the venous application length, making placement and use of a single sized cross-sectional sized stent problematic. One method to accommodate the different lumen diameters in the aortic system is with a modular stent system as shown in U.S. Pat. No. 6,193,745, hereby incorporated by reference. However, in this stent system, compressive/expansive forces on one modular section tend to shorten or lengthen the particular section, allowing for relative movement between adjacent modular stent sections, not desired in a venous system application. Such movement is not desirable, particularly where proper stent placement is critical to accommodate intersecting veins. Additionally, this system is designed for aneurysms, and hence, liners are employed.
In order to overcome some or all of these drawbacks, a stent system is needed that can account for the difficulties of placement within the venous system and to accommodate variations in geometry along body lumens without compromising the effectiveness of the stent. It would further be desirable to provide adaptable modular stents and methods for their placement which would facilitate effective treatment of widely varying luminal system geometries without requiring the maintenance of a large inventory of stent module models.
2. Summary of the Invention
The present invention provides modular intraluminal tubular stents for deployment in the venous system. The stents can be utilized in a modular system, allowing placement of multiple overlapping stents to form a composite stent structure having characteristics which are tailored to the specific requirements of the patient. A particular modular stent may have an opening or fenestra in the sidewall to accommodate flow from a side vein that joins the vein where the modular stent is positioned. A modular stent may have reinforced portions where the stent side wall material is varied for particular reasons, for instance, to add reinforcement to a portion of the stent that is subject to greater focal compressive/expansive radial body forces (a compressive force is one applied that tends to bend the stent or collapse the stent inwardly). Additionally, a stent may have a reinforced terminal end to provide additional expansive force to maintain the initial deployed location of the stent. The reinforced section extends from near one end of the stent to suitable distance back from that end, for instance form several mm to 40 mm, but generally not more than ¼ or at most, ½ the length of the stent. Modification of the construction of stent sidewall materials (such as by the addition of additional reinforcing expansive rings or variable geometry) can be made in desired portions of the stent to provide a “customized” modular stent section that can have varying axial (lengthwise) properties, or a helical expansive ring may be joined with the stent (e.g. attached to the stents interior or exterior sidewall, or interwoven into the side wall). A separate reinforcement stent (a small stent having either greater expansion characteristics or greater resistance to compressive forces) or a separate helical coil may also be used in conjunction with a venous stent or venous stent module. Stent modules may be combined to form longer stent structures as needed to fit the needs of individual patients.